Method of coating tin coated copper with indium



Patented ay 9,1944

Albert J. Phillips, Plainfield, and Albert A. Smith, Jr., Metuchen, N. 3., assignors to American smelting and Refining Company, New York, N. Y., a corporation of New Jersey No Drawing. Application May 1.4, 1941,

Serial No. 393,410

v 1 Claim. ((31. 204-435) This invention relates generally to metal or composite metal articles having a non-porous protective surface coating or integral layer andthe method of producing the same. Specifically the.

invention concerns the treatment of a metal surface to provide it with a continuous layer or coating which is free from pinholes and completely covers the base metal therebeneath.

The invention will be described more particularly in connection with the making of non-porous tin and terne plate, but it will be obvious openings in metal surfaces can be closed by rubbing or smearing indium or an equivalent metal over the surface and into the pores and imperfections. Preferably indium is used, however an alloy of indium may be emp oye or metal which is plastic enough at ordinary room temperature to be smeared into the minute openings or pores so as 'to seal them. In general metals having a Brinell hardness between and 8 may be used.

that this invention is applicable for treating various types of metal or metal alloy products to eliminate pores or minute openings in the surface making it corrosion resistant.

It. is well known that it is practically impossible, in spite of the utmost care, to make a perfectly continuous metal coating of tin on iron, steel, copper or the like metal. This is also true for terne coatings. When the metal coating contains imperfections, such as minute openings or pinholes, the base metal, of course, is not protected and in use is exposed to attack resulting in the ultimate failure of the material. Paint and lacquer has been used heretofore to fill up the pinholes and prevent corrosion but this treatment, at best, is merely temporary and does not satisfactorily solve the problem- It is the principal object of thisinvention to provide an eflicient method of rendering metal surfaces substantially continuous and free from pores or minute openings.

Another object is the production of a metal or composite article having anon-porous protectiv'e surface layer comprising indium.

A further object is the provision of a metal product wherein at least a portion of the surface is provided with a uniform continuous'coating or homogeneous alloy of indium.

These and other objects and advantages will become apparentas the description proceeds and spoils the taste of milk. It has been proposed to improve the continuity of the tin by electrotinning over a hot dipped coating and use has been made of nickel-with zinc or tin coatings followed by a heat treatment, but none of these methods have solved the difiicult problem" of producing a satisfactory coating.

we have discovered that pores and pinhole The method of closing pinholes in metal surfaces by smearing with indium, as shown by tests, is remarkably eflicient. To determine the efiect of applying indium as a protective coating, a 2-inch length of tinned copper tubing was plated with 0.0001 inch of indium over a 1-inch portion. After plating, the indium, which is easil spread by rubbing with the finger, was smeared by number of samples treated in this manner were tested and it was found that the application of indium as described was very efiective in eliminating pinholes. By this method a copper tubing was provided having a non-porous tin-indium coating which was corrosion and wear resistant.

Further tests were made to determine the elliciency of the indium smeared metal coating under corrosive conditions. For making these tests a 1% lactic acid solution was used.

' Samples of tinned copper tubes were plated on the inside with indium and then smeared by means of a soft cloth held on a stick. The tubes were then stoppered at one end and filled with lactic acid solution. For comparison an ordinary I tinned copper tube, as received, was tested along with a tinned copper tubing having an electroplated coating of tin. Spectrograph analyses were made at intervals.

Below are given the results of the tests:

v Test Spectrographicresults Coating time inhrs. Cu Sn In Regular filmed 5% 5.??3531111: ffit'if: 0.00003" In on tinned 24 Faint tracc- NiL-.. Strong. use-c t... 2.

- s on e 0---- o. tube-smeared. 72 Faint trace.. do Do. 0.00012 Sn on tinned 24 Strong Str0ngtube-smeared. 72 do do The test results clearly indicate the remark able efiect produced by the use of iridium as compared with tin and further that indium coatings of 0.00003" thickness were equally as good as an indium coating four times as thick;

Regular tinned tube..-

After the above tests were finished the tubes were split and examined. The tin coatings were heavily etchedwhifi the indium coated samples were substantially unchecked.

Another set of tests were conducted under more or less the same conditions except that in one case the indium was smeare'd? at once, in another .case seven days after plating'and in the third case not "smeared" at all. A'fourth sample had indium plated directly on copper with no under-coat of tin. As before a regular tinned tube was tested for comparison. tables give the results of this test:

. Spectrographic results I Coating 0.00010 In on tinned tube smeared at 01109.

0.00011 In on tinned tu not smeared.

l M. 8. moderately strong.

Coating 0.00011 In on tinned tube A smeared 7 days after plating. 0.000l In on plain i l mneared at once.-

The last series of tests show that rubbing or smearingof the indium is an essential stepin order to produce the improved results. Moreover it is relatively immaterial whether the indium is smeared" at once or not. The test resultsalso seem to indicate that copper builds up to a certain amount and then no further and in some instances actually decreases. In this connection it is probable that tin and indium replace copper from solution as might be expected due to their much higher position in the electromotive series of elements.

,'I'es ts made to determinethe affect of indium on'milk showed it to have little .or no effect.

The following 1 and hence is safe to When indium is plated on top or copper, tin ormetal produces a homogeneous alloy which is substantially non-porous and is resistant to corand indium which is free dium on the base metal Moreover it" is known that indium is non-toxic place in contact'with foods.

other metals and then rubbed or smeared, the pinholes and imperfections in the metal are filled producing a uniform coating. In the course of time the interdifiusion of the indium and base rosion.

.The term smearing cation and claim refers as used in the specifito overspreading of insuriace to form a continuo'us unbroken layer. The "smearing" operation is distinguished from the conventional metal surface treatments such as grinding, Dolishing and buihng in that substantially less pressure is employed than in performing the latter operations and the object is to merely spread the metal over the surface rather than remove it.

'As a modification of the processindium may be plated or otherwise coated onto the surface of the, base metal after which it is rubbed or smeared to fill imperfections in the base metal and then an outer coating, i. e., tin, zinc, etc; applied over the indium. In each case the finished product comprises, a pore sealing layer of indium which is essential.

It ,will be understood that this invention is not limited to the specific materials and uses given and that widely different embodiments of the invention may be made without departing from the spirit and s ope thereoi' as defined in the claim.

What is claimed is:

The method of treating tin coated copper articles used for handling milk whereby small minute openings in the tin coating are, sealed preventing milk from coming-into contact with the copper base metal therebeneath, which consists in plating indium on said tin coating to a thickness of 0.00003 inch, thereafter rubbing the indium coating with a soft cloth so as to spread the indium into surface imperfections in the tin coatin'g filling them -so that the copper base metal is provided with a sealing layer consisting of tin from. pinholes and misuch as would expose the base ALBERT .1. PHILLIPS.

sauna-r a; sm'rn, JR.

nute openings metal. 0 

